1. Field of the Invention
The present invention relates to a spectrum information measurement method, a color sensor, and a virtual slide device.
Priority is claimed on Japanese Patent Application No. 2010-215905, filed Sep. 27, 2010, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
A reading circuit that reads at a high level of sensitivity while removing switching noise at the same time is disclosed in Japanese Unexamined Patent Application, First Publication No. 2007-336157 as an example of a reading circuit of a color sensor that is used to acquire spectrum information about a test specimen. The structure of a conventionally known solid-state imaging device will now be described with reference made to FIG. 8. FIG. 8 is a schematic view illustrating the structure of a conventionally known solid-state imaging device. In the example shown in the drawing, a solid-state imaging device 100 is formed by an integrated circuit unit B1, a CDS (Correlated Double Sampling) circuit unit B2, and an S/H (Sample Hold) circuit unit B3.
In the integrated circuit unit B1, an anode of a photodiode 10 that is used to receive light and generate photoelectric current is connected to a non-inverting input terminal of an operational amplifier 50, while a cathode of this photodiode 10 is connected to an inverting input terminal of the operational amplifier 50. The non-inverting input terminal of the operational amplifier 50 is connected to a reference voltage supply 20. In addition, an integrating capacitor 40 that is used to accumulate photoelectric current and a switching device 30 that is used to control the integration time are connected in parallel between the inverting input terminal and an output terminal of the operational amplifier 50.
In the CDS circuit unit B2, one end of a capacitance element 60 is connected to the output terminal of the operational amplifier 50 forming part of the pixel unit B1, while the other end of the capacitance element 60 is connected to an inverting input terminal of an operational amplifier 90. A non-inverting input terminal of the operational amplifier 90 is connected to a reference voltage supply 70. One end of a capacitance element 80 is connected to an inverting input terminal of the operational amplifier 90, while the other end of the capacitance element 80 is connected to one end of a switching device 120 and one end of a switching device 140. The other end of the switching device 120 is connected to a reference voltage supply 130, while the other end of the switching device 140 is connected to an output terminal of the operational amplifier 90. One end of a switching device 110 is connected to one end of the capacitance element 80 and to a connection point between the inverting input terminal of the operational amplifier 90 and the capacitance element 60, while the other end of the switching device 110 is connected to the output terminal of the operational amplifier 90.
In the S/H circuit unit B3, one end of a switching element 150 is connected to the output terminal of the operational amplifier 90 forming part of the CDS circuit unit B2, while the other end of the switching element 150 is connected to a non-inverting input terminal of an operational amplifier 170. One end of a sample hold capacitance element 160 is connected to the non-inverting input terminal of the operational amplifier 170, while the other end of the capacitance element 160 is grounded. A signal output terminal 180 connects together the inverting input terminal and the output terminal of the operational amplifier 170, and is connected to the output terminal of the operational amplifier 170.
Operations of the solid-state imaging device will now be described with reference made to the timing chart shown in FIG. 9. FIG. 9 is a timing chart illustrating the operation timings of a conventionally known solid-state imaging device 100. On this timing chart, the respective switching devices are in a conductive state in the High-level intervals on the chart, and are in a non-conductive state in the Low-level intervals on the chart. φR shows the switch control timing of a switching device 30, φRC shows the switch control timings of switching devices 110 and 120, φT shows the switch control timing of a switching device 140, and φSH shows the switch control timing of a switching device 150. A voltage V1 shows the voltage of the output terminal of the operational amplifier 50, while a voltage V2 shows the voltage of the output terminal of the operational amplifier 90, and a voltage Vout shows the voltage of the signal output terminal 180. Four time periods, namely, T1 through T4 are formed in the time axis direction.
The time period T1 is a reset period, and φR, φRC, and φSH are set to a High state, while φT is set to a Low state. In the time period T1, the voltage V1 changes to the voltage Vr1 of the reference voltage supply 20, the voltage V2 changes to the voltage Vr2 of the reference voltage supply 70, and the voltage Vout is equivalent to the voltage Vr2 of the output terminal of the CDS circuit unit B2.
In the time period T2, φRC and φSH are set to a High state, while φR and φT are set to a Low state. Photoelectric current generated by the photodiode 10 is accumulated in the capacitance element 40. At this time, if the elapsed time from the point when φRC was first set to a High state is taken as TINTGW, then the voltage V1 of the output terminal of the pixel unit B1 is shown by the following Formula (1).V1=Vr1+(Ipd×TINTGW)/C0  (1)
Here, the value of the capacitance of the capacitance element 40 is C0, the amount of photoelectric current generated by the photodiode 10 is Ipd, and the voltage of the reference voltage supply 20 is Vr1.
However, in actual fact, clock feedthrough which is caused by the switching operations of the switching device 30 is superimposed on the voltage V1 of the output terminal of the pixel unit B1. As a result, the voltage V1 changes in the manner shown in Formula (2).V1=Vr1+(Ipd×TINTGW)/C0+Vn  (2)
Here, the voltage changes caused by the clock feedthrough unit are shown as Vn.
In the time period T3, φT and φSH are set to a High state, while φR and φRC are set to a Low state. At this time, the voltage V1 of the output terminal of the pixel unit B1 is shown by the following Formula (3).V1=Vr1+(Ipd×TINTG)/C0+Vn  (3)
Here, the elapsed time from the point when φR and φRC were first set to a Low state is taken as TINT.
In this period, the switching devices 140 and 150 are in a conductive state, while the switching devices 110 and 120 are in a non-conductive state, and the voltage V2 of the output terminal of the CDS circuit unit B2 temporarily changes to the voltage Vr3 of the reference voltage supply 130. Thereafter, because the operational amplifier 90 and the capacitance elements 60 and 80 make up a charge amplifier circuit, the voltage V2 of the output terminal of the CDS circuit unit B2 can be shown by Formula (4).V2=Vr3−(C1/C2)×(Ipd×TINTG)/C0  (4)
Here, the value of the capacitance of the capacitance element 60 is taken as C1, while the value of the capacitance of the capacitance element 80 is taken as C2.
During this period, the switching device 150 is in a conductive state, and the operational amplifier 170 forms a voltage follower circuit. In addition, the voltage Vout of the signal output terminal 180 has the same voltage as the voltage V2 of the output terminal of the CDS circuit unit B2. Accordingly, the voltage Vout of the signal output terminal 180 is shown by the following Formula (5).Vout=Vr3−(C1/C2)×(Ipd×TINTG)/C0  (5)
As a result of the operations during this period, the clock feedthrough voltage Vn which is caused by the switching operations of the switching device 30 can be removed.
In the time period T4, φR and φRC are set to a High state, while φT and φSH are set to a Low state. The switching device 150 is in a non-conductive state, and the voltage shown by Formula (5) is maintained in the signal output terminal 180. It is possible for the signal to be amplified by the capacitance ratio of the capacitance element of the CDS circuit unit B2 and then read. Any reset noise caused by the switching operations of the switching device 30 which is connected to the capacitance element 40 of the pixel unit B1 can be removed by a correlated double reading of the CDS circuit unit B2.
A conventionally known solid-state imaging device can be used as a color sensor. FIG. 10 is a schematic view illustrating the structure of a color sensor to which a conventionally known solid-state imaging device has been applied to. In a color sensor 200 shown in the drawing, a circuit corresponding to the integrated circuit unit B1 of the conventionally known solid-state imaging device 100 is shown as an integrated circuit unit B10, while a circuit corresponding to the CDS integrated circuit unit B2 of the conventionally known solid-state imaging device 100 is shown as an integrated circuit unit B20. Note that there is no depiction of any circuit that corresponds to the conventionally known S/D circuit unit B3.
In the example shown in the drawing, the color sensor 200 includes integrated circuit units B10-1 to B10-6, gain circuits B20-1 to B20-6, integration time calculation units 38-1 to 38-6, gain calculation units 39-1 to 39-6, and a drive control circuit 310. The integrated circuit units B10-1 to B10-6 include pixels 31-1 to 31-6 that detect spectrum information about a subject by dividing it into respective wavelength transmission bands, reference voltage terminals 32-1 to 32-6, switching elements 33-1 to 33-6, capacitance elements 34-1 to 34-6, and operational amplifiers 35-1 to 35-6. Portions formed by the reference voltage terminals 32-1 to 32-6, switching elements 33-1 to 33-6, capacitance elements 34-1 to 34-6, and operational amplifiers 35-1 to 35-6 are called read circuits 30-1 to 30-6.
In the drawing an example is shown in which the pixels 31-1 to 31-6 provided in the integrated circuit units B10-1 to B10-6 detect six colors, namely, violet, blue, green, yellow, red, and orange. Specifically, the pixel 31-1 provided in the integrated circuit unit B10-1 is a pixel that detects violet light. The pixel 31-2 provided in the integrated circuit unit B10-2 is a pixel that detects blue light. The pixel 31-3 provided in the integrated circuit unit B10-3 is a pixel that detects green light. The pixel 31-4 provided in the integrated circuit unit B10-4 is a pixel that detects yellow light. The pixel 31-5 provided in the integrated circuit unit B10-5 is a pixel that detects red light. The pixel 31-6 provided in the integrated circuit unit B10-6 is a pixel that detects orange light.
In the color sensor 200, light from a subject is irradiated onto the pixels 31-1 to 31-6. The color sensor 200 also controls the integration time in the switching elements 33-1 to 33-6 using as a reference a reference voltage which is applied to the reference voltage terminals 32-1 to 32-6, and integrates the light from the subject as voltage changes that correspond to the photoelectric current in the capacitance elements 34-1 to 34-6. It then outputs the results to output terminals of the operational amplifiers 35-1 to 35-6.
The color sensor 200 amplifies output changes from the output terminals of the operational amplifiers 35-1 to 35-6 using the gain circuits 36-1 to 36-6, and then reads them. The integration times of each of the integrated circuit units B10-1 to B10-6 are calculated by the integration time calculation units 38-1 to 38-6 using information sent from the drive control circuit 310. The gains of the respective gain circuits 36-1 to 36-6 are calculated by the gain calculation units 39-1 to 39-6 using information sent from the drive control circuit 310. As a result of this, output signals are output from the output terminals 37-1 to 37-6 for the integration time and the gain that are set by the integration time calculation units 38-1 to 38-6 and the gain calculation units 39-1 to 39-6.
The spectral characteristics of a multiband color sensor will now be described. FIG. 11 is a graph illustrating the spectral characteristics of a multiband color sensor that is formed by coating color filters on the front surface of a light receiving element (i.e., a photodiode or pixel) of a light sensor in order to detect spectrum information about a test specimen. This graph shows a curve 2001 that shows the transmittance of a color filter that has been coated on the front surface of a color sensor that detects violet light, a curve 2002 that shows the transmittance of a color filter that has been coated on the front surface of the color sensor that detects blue light, a curve 2003 that shows the transmittance of a color filter that has been coated on the front surface of the color sensor that detects green light, a curve 2004 that shows the transmittance of a color filter that has been coated on the front surface of the color sensor that detects yellow light, and a curve 2005 that shows the transmittance of a color filter that has been coated on the front surface of the color sensor that detects red light. In this manner, the wavelengths of the light transmitted through each color filter differ in accordance with the color of the detected light.
FIGS. 12A and 12B are timing charts illustrating the operation timings of a color sensor 200 to which a conventionally known solid-state imaging device has been applied. FIG. 12A is the timing chart obtained when the color sensor 200 acquires spectrum information normally. FIG. 12B is the timing chart obtained when a fixed quantity of light or more is irradiated onto the color sensor 200.
If less than the fixed quantity of light is irradiated onto the color sensor 200, then in the same way as was described using the timing chart illustrating the operation timings of the conventionally known solid-state imaging device 100 shown in FIG. 9, as is shown in FIG. 12A, the color sensor 200 is able to acquire spectrum information normally.
However, when a fixed quantity or more of light is irradiated onto a specific pixel in a sensor having spectral characteristics such as those shown in FIG. 11, then as is shown in FIG. 12B, V1 becomes saturated in the time period T2. At this time, because there are no voltage changes in the time period T3, the final output voltage V2 changes to zero and shows a false value. It is not possible to determine in this case whether the reference voltage was output with the zero changes in voltage being due to there being few irradiated wavelength components, or whether the reference voltage was output when saturation was reached in the time period T2 as a result of a fixed amount of light or more being irradiated.
FIG. 13 is a graph illustrating a relationship between the amount of light and the output when the output from the gain circuits 36-1 to 36-6 dropped to zero when a fixed quantity or more of light was irradiated onto the color sensor 200. The horizontal axis in the graph shows the amount of light, while the vertical axis shows the output from the gain circuits 36-1 to 36-6. As is shown in the drawing, when the amount of light of the wavelength component irradiated onto a particular pixel of the color sensor 200 was a fixed amount of light or more, the output of the gain circuits 36-1 to 36-6 did not reach the saturation level output which is shown in the graph by the dotted line, and as is shown by the solid line, there was no saturation output and the amount of irradiated light dropped to zero. Because of this, false spectrum information is acquired by the color sensor 200, and it is not possible for accurate spectrum information to be acquired.